Graymails are defined as solicited bulk mail messages that don't fit the definition of spam. Graymails might have been opted into by a recipient but the recipient is no longer interested in the emails. In addition, an email can be considered as graymail for one user but that could be an important email for another user of the same organization. For example, a person buying a house may subscribe to receive mailing related to furniture as the person considers buying furniture for the new house. Once the house and the furniture is bought, the person typically has less interest in the emails about buying furniture, and those emails become graymail for that person. Another person, however, may at that point just be starting to buy some furniture, so furniture-related emails for that other person might not be graymail. Thus, the classification as graymail or non-graymail is user-dependent.
Hence, filtering graymail is a major problem for organizations because even an optimal filter will perform unsatisfactorily unless user preferences are taken into account. Existing methods of graymail detection and filtering explicitly ask users to report graymail. However, these methods might not be scalable as recipients often find providing feedback on each graymail an additional nuisance and simply do not reliably report receiving graymail.